


A Difference in Size

by FullOfBoredom



Category: Undertale (Video Game)
Genre: Bara Sans (Undertale), Come Inflation, Inflation, M/M, Red is smol, Size Difference, Size Kink, cumflation, kustard - Freeform
Language: English
Status: Completed
Published: 2021-01-24
Updated: 2021-01-24
Packaged: 2021-03-16 02:35:23
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,361
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28949049
Author URL: https://archiveofourown.org/users/FullOfBoredom/pseuds/FullOfBoredom
Summary: “Are you sure about this Red?” Sans stared down at his very nervous boyfriend. Red took a few deep breaths, backing up to look up at his task at hand.Sans was almost twice his height, and twice his width. Sans’s hands covered half of his ribcage. Sitting in his lap, Sans could kiss the top of his head, but only if he bent his neck down. Kissing him was almost suffocating at times, and Red’s attempts at oral had always ended in something that Sans found adorable but Red found frustrating.
Relationships: Sans/Sans (Undertale)
Comments: 2
Kudos: 64





	A Difference in Size

“Are you sure about this Red?” Sans stared down at his very nervous boyfriend. Red took a few deep breaths, backing up to look up at his task at hand.

Sans was almost twice his height, and twice his width. Sans’s hands covered half of his ribcage. Sitting in his lap, Sans could kiss the top of his head, but only if he bent his neck down. Kissing him was almost suffocating at times, and Red’s attempts at oral had always ended in something that Sans found adorable but Red found frustrating.

“Yeah, I just gotta work up to it.” Red climbed up into Sans’s lap, determined that today would be the day. He’d finally please Sans the way he pleased Red. 

“I’m flattered you wanna Red, but I told you, I’m fine just playing with you.” Sans cupped Red’s cheek (well, head), and Red leaned into it. “It’s not like I’m dating you for sex.”

“I know. But you do this for me all the time. If I can do this, then maybe…” Red blushed, shaking at the thought of what he was going to attempt. “I can please you even half as well as you please me.” Red kissed the hand, then pushed it away to take off his jacket.

“You already do darling, but if you’re sure, we’ll take it nice and slow.” Sans helped Red remove his clothes, tossing them to the side of the bed. They were at Sans’s place, and Sans was only in a T-shirt and boxers, having almost gone to bed when his boyfriend had come over. Red stripped down to just his boxers, standing up on Sans’s thighs to drop them.

“Oh, _babe_.” Sans stared down at the delicious display before him.

Red put his hands on Sans’s shoulders, glorious red ecto covering his torso, pelvis, and parts of his limbs. He could practically taste Red from here, staring at the wet pussy between his legs. Standing like this put him just above Sans’s face; it was fun to lean down and be kissed for a change.

Red groaned at Sans’s thick tongue working its way into his mouth. Red couldn’t really run out of air, no lungs, but this did make him feel lightheaded. Sans pulled back when the hands on his body clutched his collarbone tightly.

“You back on earth love?” Sans half lidded gaze raked over the cute red blush on Red’s face.

“Mmhmm. Where do we start?” 

“With a snack.” Red was flipped onto the bed, Sans pinning him with one hand, not that he needed to hold Red down, just hold him still.

“This is supposed to be about you!” Red tried to get up, but fell back when Sans put his thick finger directly inside him. “Saaaaaans.”

“You need some prep if I’m even gunna humor your idea.” He wiggled it around, chuckling at Red’s moans and then gasps when he licked his clit. Red bucked and whimpered, only ceasing when Sans replaced the finger with his tongue.

“You’re gunna need to f-f-f-ffffffffff-AHHHHHHH.” His tongue finally really got going, slurping up some of his favorite flavor. “FUCK! Fit three fucking fingers!”

Sans couldn’t respond, his mouth was full, but he did look up at Red’s face with glowing eyes, holding his ribs down enough to cause Red to squirm. He tapped each of his three middle fingers in turn, letting Red feel their weight and size, turning him on even more.

Sans lapped up all of Red’s juices spacing out staring out his cute, overwhelmed face.

Sans honestly never expected to be able to have Red. He was a pretty big guy, and he knew Red was terrified of him at first. Red hadn’t lived this long trusting huge people who could crush him. And even when he had fallen for the cute face who also loved jokes and science, he wasn’t sure that Red would accept him that way. AND THEN, when he did, his main concern was that he couldn’t sexually please Sans with his own body. Sans couldn’t give a damn, he got more than enough in exchange.

There was a small part of him though that was desperately curious to see if he could though. 

He pulled his tongue out when Red laid bonelessly moaning and sensitive. The two fingers fit without much complaint.

“Saaaaans…”Red thrusted against the large fingers, groaning and trying to shove them deeper just with canting from his pelvis. “More, more of you, more!”

“If you want it that bad, we’ll step it up.” The third was a tight fit, but Red’s mind was too gone to mind. Red had taken three fingers before, he had bigger goals today.

“Oh fuck, Sans, fuck, I want it so baaad.” Red gasped and thrust recklessly against them. God, the stretching drove him wild. Red didn’t know if he could be satisfied with anything less ever again. Taking Sans could ruin him, make him unsateable for life by anyone else, but he was in too deep, too desperate to please the love of his life.

“Fuck Red, I’m gunna add a fourth. You’re doing so good babe.” Sans could feel his own erection trying to escape his boxers. Red might be able to, holy fuck. Even if he couldn’t, the _idea_ that he _might_ threatened to drive Sans insane.

“AAAAAAAAAAH!” Red’s body convulsed at the width it could fit, mind going blank at the resistance of his ecto to the intruding fingers. Sans only kept finger fucking him under he got a clean slide in and out without Red groaning in discomfort. He left them inside Red, waiting until Red’s brain clicked on enough to actually hear him, to speak back.

“Red, earth to Red. You’re sexy as fuck and I’d go to hell for you, over.”

“Hello earth, this is Red, over. I’m…” Red looked down to his face, meeting Sans’s eyes and taking a huge gulp of air. “…ready to come in for a landing. Over.”

“Red, are you-” Sans felt his soul flip around, suddenly lying on his back. Red whimpered at the fingers being gone, managing to get around his large boyfriend and climb on top of Sans, dropping directly over the rather large bulge in his shorts.

He grabbed at Sans’s waistband, pulling down until he finally set the bulge free.

The thick blue ectocock stood about as tall as his sternum, the huge tip widening down to a width that exceeded the four fingers he had taken earlier. He stood up, straddling Sans’s waist, adjusting just right to have the tip touch his waiting entrance.

“Safe word is Orion.” Red, nervously licking his teeth and lightly rubbing the tip against himself, locked eyes with Sans.

Sans looked incredulous, clearly trying to burn this into his memory. His cock twitched at the thought, Red forced to sway with the weight of it. He leaned onto it, just until it began to stretch him further than the fingers had.

“I…can do this…” Red dropped a little further. He was so, so full. How much had he taken?

After a quick glance down, he determined a third was in, two thirds to go. He’d need to put more weight into it to even attempt to sink down.

Sans was losing his fucking mind. Red’s face glowed as bright as their ectos, even only a third of the way down his cock. That’s as far as he got before he paused. He could see the very clear outline of dick right through Red’s body. Oh, this was so worth it, maybe Red would let him just fuck him with the tip, because this was too good to walk aw-

“ _holy fucking **shit** Red_”

Red had gotten down to kneeling on Sans’s lap, two thirds now, and stuck just above the thickest part of the cock. He was as far up as kneeling could leave him.

“You’re so fucking thick Sans, I can barely take iiiiiiiiit!” Red’s mouth kept opening and closing, huffing out all the breath in his body. The line of Sans’s dick glowed fiercely in him, definitely protruding far above his pelvis and bulging his stomach heavily to fit so much inside of him. His brain fuzzed over, focused solely on the burn of the stretch and the thought of what he was fucking himself apart with. He turned to Sans, and unintentionally tightened around the huge thing inside him.

Sans always kept his composure pretty well around Red, Red the one to always blush and be embarrassed by all the antics and romance and flirting. 

His boyfriend looked half feral. Sans was staring him down like others might stare at works of art in a museum. Completely in awe, swept up in overpowering feelings, desperately in love. Red moaned.

“Sans!” Red reached for him, and Sans shakily lifted his hands to cradle Red’s face, wiping away the tears gathering in the corner of Red’s eyes. Red grabbed the hands, his own shaking so bad that Sans would worry if not for the fact that Red had declared a safe word beforehand. Red pulled them off his face, and guided them to his hips. Red sank a little further, but his body resisted even under the weight of them.

“Do it.” Red pressed down on his arms. 

“Red, you don’t have to do this.” Sans rubbed his fingers in soothing circles on Red’s waist, breathing like he ran a marathon. There weren’t many monsters even close to his size, he hadn’t got to have even this much with anyone before. Red looked stretched to the brink, his body struggling to contain him. He felt so guilty, but he didn’t deny that he wanted to _grab those hips and ride out everything Red could take without physically breaking._

But maybe Red could hear thoughts, because he just glared at Sans.

“Yes I do. Do it Sans.” And Sans’s control was shot to hell, because he did.

He pushed Red down, slowly but steady, and didn’t stop until they were pressed flush together. Sans held him there, because any movement whatsoever and he’d pound the life of his poor little boyfriend.

Red’s eyelights had poofed out of his face, mouth open in a silent cry. The cock sat so high in his body, overstuffed and overstimulated. He had to breath through the feeling of containing more than his body was meant to hold. But he wanted this, he wanted Sans, he needed to do this for him. Sans’s eyelights had gone into full hearts.

“Fuck babe, I love you so damn much.” Sans released him a little, letting Red bounce only an inch, but feeling the way the thrust would shake his body. “Red, baby, please, talk to me, fuck.”

“Just, go slow.” Red shuddered when Sans thrust into him, a few inches this time. He gradually increased the length of the thrust, keeping the speed slow. 

Sans had to stare directly at Red’s face. It took every single ounce of self control he could muster to not slam into his ridiculously stretched pussy, to hear every noise he could drag out of that cute little mouth. He wasn’t sure how long he could keep this up. Hopefully as long as Red needed him to.

“Red, baby, I love you, you’re so tight, I can’t believe you took all of me, you’re incredible, I love you.” Sans’s pace increased, still gradual, but clearly gaining momentum.

Red could feel the safe word on the edge of his tongue, but swallowed the urge. His body bulged and shook under the massive cock pistoning in and out of him, pounding his abused magic, starting to actually slam into him. Sans grip on his hips turned iron as Sans essentially jacked himself off with Red’s body. Red’s hands laid limply by his sides, but he brought them up to instead rub over his stomach. He glanced down at the huge dick his body was taking for Sans, his lovely boyfriend whom his soul had already sold itself to. He pulled himself off of the edge of the safe word by focusing on what was happening.

He’d been stretched wide. He had taken Sans’s cock down to the hilt. He was going to take Sans’s load deep into his body. He was going to satisfy his boyfriend, the love of his life, in a way that no one else had managed to before. He’d said no one could do more than a very enthusiastic handjob for him. That technically made this taking Sans’s _virginity_. And that was more than enough for him to slip back into the mood. One hand rubbing over the rapidly appearing and disappearing bulge, another straying down to his clit and nearly screamed for touching it.

“ _fuck_ ” Sans sped the fuck up, needing to get to the peak, to be there with Red. He openly stared at his gorgeous boyfriend, taking him without a single complaint, rubbing himself off on the experience of being fucked apart by his boyfriend’s huge cock. Red was getting tighter, Sans’s rhythm off beat, he called out for him.

“Come _on_ Red, I love you, fucking, _come for me_!”

“SAAAAAAANS!” Red screamed so loud Sans almost flinched, but the second he felt that clench, he followed Red directly into freefall.

Red’s orgasm jumped through his body like a live wire. Every nerve lit up, burning and warming in turn, while he could feel his core getting hot and full and Red couldn’t think about anything else for minutes, mentally spaced and physically sagging against Sans, unable to do anything else but breathe raggedly and wait patiently to come down.

When he did come to, he tiredly looked down at his body.

It was glowing purple. All of Sans’s cum shone through his red ecto as purple magic. Sans hadn’t pulled out of him yet, so he looked up at him.

Sans’s eyes were closed. He looked as exhausted as Red felt.

“Sansy?…” Sans snapped to attention. He steadied him with an arm and cradled his skull with the other hand.

“Are you okay?” Red tightened experimentally around the softer cock inside him. Sans sighed happily, but searched Red’s face for any sign to pull out.

“Yeah, just sore.” He rubbed his stomach. “And very full.”

“You look good like that.” Sans rubbed a hand over his taut stomach. It felt warm and comfortable. It soothed some of the ache.

“I look pregnant.” Sans paused, choosing his words carefully.

“I…don’t retract my previous statement.” His face went bright bright red. And then he murmured softly.

“It’s possible…” Sans, in awe and so, so happy, smiled at him. 

“I love you.” He bent himself down to clank Red’s head. “You’re incredible.”

“Nah, that’s you.” Red stretched his arm up and back, looking back down at his stuffed belly. If Sans pulled out here, the bed wouldn’t be able to be slept on. It’d also probably ruin the carpet. “Though for such an incredible pair, we didn’t think too far ahead on this one. How do we not fuck up all the surrounding area when you pull out?”

Sans turned bright blue.

“Honestly, I didn’t think you’d be able to…”He hurriedly explained more. “I mean, no one’s ever even tried. It’s apparently intimidating up close.” Red chuckled at Sans looking so flustered.

“You shouldn’t have underestimated me.” Red sighed, feeling up his body and marvelling at the warmth and size again. The only reason Sans’s dick hadn’t slid out by now was solely it’s girth, Red’s body couldn’t push it out even after tightening so much. “Though it’s not like I made a plan either.”

Then Red had an idea. A fucking crazy idea, his body reminded him, but he did have one.

“Hey Sans?” Sans paused his current soft exploration of Red’s ectoflesh, still in awe, to look at his face.

“Yeah love?”

“How do you feel about a round two?” Oh, Red wanted to frame that face. Sans eyes had gone _dark_ , and he had felt a pulse from the cock still inside him.

“Red, you really want…”Sans chuckled, low and deep, rumbling his chest and Red along with it. “You’re something else.” Red could already feel his plan working, even if his body groaned at being used so soon, so extremely.

“Listen, if we move, I’m gunna leave a gross splash zone in the bedroom. But only if we move while you’re soft.” Red wiggled his hips a little. “Think you could get it up again? Even if we can’t really move yet?”

“Could I?” Sans’s voice dropped an octave. “Babe, you’re still speared on me, filled with my magic. You’re goddamn gorgeous, and you’re asking for more?” Red felt Sans really getting into it. “You take my fucking monster cock like a champ, and you still want more of it?” He stroked all over Red’s body, growling at Red’s little whimpers of pleasure. “You’re some kinda freak, aren’t ya?”

“Only for you Sansy.” Red kept his body grounded by holding onto Sans’s ribs. He left little scratches from holding on so tight.

“Of course ya are. Now that I know you can take it, you think I’m letting you go?” Sans was already half hard again. Before, it was a pipe dream to have Red once, and now he might get to fuck him _twice in the same nigh_ t. “I don’t need any motivation sweetheart, just gotta wait out the clock until my body will let me.”

Red shivered at the insinuation, even though he still felt plenty warm. Sans’s dick was already starting to take up a lot of space. His walls sent out bites of pain, but they were soothed by all the lubrication still sloshing around inside him. Part of him worried that even suggesting this would set a precedent that Sans could have him however many times he wanted, the only prerequisite of there being enough time to prep for the first go. 

The other part of him hoped Sans would take that hint and run with it.

“We - mmmmphh - gotta move before you go too wild, big guy.” Red could feel that Sans fully recovered, his body still freshly aching from the _wonderful_ scrambling of his insides 15 minutes ago. So thick, so hot. Red let his thoughts go, instead just focusing on every single feeling his body had about Sans’s intrusion.

Sans nodded, but honestly couldn’t even think of how to go about it. This was gunna be awkward, even with his size.

He put an arm under Red’s back, focusing mainly on his rib cage, and made sure to push their pelvises firmly together to keep most of the cum inside. He stood up off the bed, using both hands to make sure Red was steady first, and then slowly walking towards the door. Where to go? The only place without carpet was the kitchen/dining room and bathroom, and he didn’t know how Red would feel about either of those.

Then Sans had a crazy idea, and walked them both to the dining room.

The table was kinda low for Sans, practically just better than a coffee table; he really wanted to replace the stupid thing, but now, it would serve his purposes effectively. He moved right up to the table, grabbed Red with both hands, and then _spun him around_.

Red suddenly felt sturdy wood beneath his hands and knees. He whined at the sensation of being flipped. And then a large hand dropped on his back.

“You feeling okay?” Red couldn’t breathe. Sans’s cock, at this angle, already sunk lower into his ribs. He’d be knocking up against his soul at this rate. Not to mention, he was already stuffed full. Sans hadn’t pulled out and emptied Red’s body at all.

“ _Saaaaans_.” It’s all he could say. His body and soul consumed by the intense connection, half of his body was taken up by Sans now. His wrecked moans were the only verbage he could muster right now. Sans didn’t want to accidentally hurt him.

“Red, what’s the safeword?”

“ _Orion_.” So he wasn’t all gone. Just very, very lost.

“Brace yourself.” 

Sans started slow, thrusts to reaquaint Red’s body with his size. He didn’t need more than a few before Red moaned back at him.

“ _More!_ ” Fuck, he’d give him more. So much more. This angle, he wasn’t moving Red with his arms, he was throwing his weight into them now. Each thrust was powerful, rocking the table Red was on, and Red only got louder. His arms had given out, letting his back curve downward, and Sans hit even deeper.

_“FUCK!” He’d tapped Red’s soul._

Sans almost panicked, but Red canted his hips backwards, trying to take more of the fucking thing into his abused pussy.

His love was a _freak_. A wild delicious little thing, who loved Sans of all people, and Sans helplessly wanted to please him.

Each thrust shook all of Red’s bones and sent shockwaves through his body. Soul sex usually didn’t work like this, but damn if Sans’s huge cock hadn’t demanded it anyway. Where was he? All he could think about was being pounded into, how full he was, the stretch, the cum still filling him to the brim, and Sans name over and over and over again.

Red’s only sounds were very loud moans (thank the stars this wasn’t an apartment), tongue lolled out, swept away by pleasure and Sans was about to follow him.

“Red, I’m so fucking close babe. Come for me and I’ll pull out.” Sans desperately needed to see and feel Red orgasm, but he didn’t want to overfill his small body and hurt him. He could cum on his back. He’d look great anyway he could have him.

 _“DON’T YOU FUCKING DARE!_ ” Red barely held on past the words. He was _gone_. All wild moans and clenching, Sans whole body fucking shook and only lasting thorugh three hard pumps before he came.

He slow thrusted through the pleasure, wave after wave spilling out of him, into Red. Sans watched him inflate even bigger than he thought possible. Stomach and body distended so much further and Red could only whine and claw at the table, endlessly repeating Sans’s name to the stars.

He came back down first, holding his small lover in his hands. Red was still shaking, a wobbly mess that got quieter and quieter until he heard one crisp clear word.

“Orion.”

Sans pulled out of him, and he couldn’t get out of the way fast enough. Red’s body gushed blue magic, some getting on the table, but mostly on the linoleum floor. Red’s knees knocked together, gush after gush dumping out of him, and once it stopped, Sans gently tipped his boyfriend into his arms, overturning him from on his hands and knees, to let even more flow out. 

Once it stopped, he pulled Red up into his chest like a teddy bear. He still had a little trapped inside, but it only made him look a little chubby.

“I love you.” He kissed him gently. “How you feeling?” Red rattled quietly against him.

“Exhausted from all the bone-ing.” Red’s eyes were heavy, but he did smile. Sans felt his soul explode in fondness. He cuddled Red close. 

“Let’s get cleaned up and sleep then, my _gush_ ing beauty.” Sans laughed when he felt Red softly hit him with his fists. He wanted to see the cute blush, but didn’t want to get too unfocused. He grabbed his hoodie, and walked back with Red, setting him on the table (a clean part) and wrapped it around him. “Gotta clean the floor. I promise I’ll come back and get you clean. Just rest.”

Red yawned and pulled the comfy hoodie tighter around him. Sans used an entire roll of paper towels, turned on the shower to heat up, and then swiffered the floor to clean the residue. It’d need a better clean later, but he was just trying to get them both into bed.

After that, he carried Red into the bathroom. He hung his hoodie by the door, carrying Red with him. 

Red focused his sleepy mind to let the ecto body go, letting the last bit of blue magic pour down the drain and on Red’s pelvis. Sans gently cleaned every inch of him (and himself) before turning the water off and pulling out some towels. 

Sans’s towels were as tall as Red, and drying him was so cute, his little head popping out the top, very warm and happy. Sans gladly let him have his hoodie back when the towel came off and Red almost shivered to death.

“Finally.” Red curled up right into Sans when they finally ended up in the bed. “I love you Sans.” He would’ve been so cold if not for Sans and his hoodie to keep him warm. He wasn’t full of warm magic anymore. Vanishing the ecto helped with the empty feeling. “You know that, right?”

Sans settled an arm around Red, touching as much of their bodies as he could. His soul glowed softly in the night, framing Red in the light.

“I definitely do now. You’re it for me. I’m yours forever.” Sans sleepily winked at him. Red hummed pleasantly.

“You’re gunna fall for the first guy you have sex with? Guess I’m lucky it was me.” Red laughed, but Sans looked serious for a second. “I know you like me for me, Sans. It’s okay.”

“I’ve been in love with you for months now. But after tonight, after actually having you, feeling your soul and seeing how far you were willing to go for me…”Sans beamed at him. “I’m yours. You can have anything you want as long as I have you. I love ya Red, I truly do.”

“Good, I’m taking half the bed.” Red blushed. “And I love you too Sansy. Honestly.” Sans pulled him up for a kiss, and they settled in with a smile. Red grinned as he fell asleep, tired and aching, but sated in his very soul. 

Well, until next week, at the very least.

**Author's Note:**

> Written as part of Kinktober.


End file.
